Stem cell quiescence has been reported in many systems, and typically involves the slowing or stalling of the cell cycle and low transcriptional activity. Primordial germ cells (PGCs) of sea urchin are known to enter a quiescent state prior to gastrulation, before re-activating later in development. Now (p. 1201), Gary Wessel and co-workers show that this quiescence also involves a significant reduction in translational activity. Two potential mechanisms are uncovered. Firstly, Nanos2, which is expressed specifically in PGCs, binds to and downregulates the critical translation factor eIF1A. Secondly, mitochondrial number and activity is low in PGCs, which might induce a switch to glycolytic metabolism and hence an acidification of the cytoplasm. Increasing cellular pH promotes translational activity specifically in PGCs. This work raises many intriguing questions. For example, how is translational activity re-activated at later stages? How are the metabolic changes in PGCs orchestrated? How general might this be in quiescent stem cell populations? Thus, the identification of this previously unrecognised phenomenon of transient translational quiescence in sea urchin PGCs opens up many new avenues for investigation.
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